1. Field of the Invention
This invention relates to an optical probe, and more particularly to an optical probe having a tubular outer envelope and having a function of emitting light from the peripheral surface thereof and an optical tomography system where such an optical probe is employed.
2. Description of the Related Art
As a conventional method for obtaining tomographic images of measurement targets, such as living tissue, a method that obtains optical tomographic images by OCT (Optical Coherence Tomography) measurement has been proposed (refer to Japanese Unexamined Patent Publication Nos. 6(1994)-165784 and 2003-139688). The OCT measurement is a type of light interference measurement method that utilizes the fact that light interference is detected only when the optical path lengths of divided light beams, that is, a measurement light beam and a reference light beam, match within a range of coherence length of a light source. That is, in this method, a low coherent light beam emitted from a light source is divided into a measuring light beam and a reference light beam, the measuring light beam is irradiated onto a measurement target, and the measurement light beam reflected by the measurement target is led to a combining means. Whereas the reference light beam is led to the combining means after its optical path length is adjusted so that its optical path length equalizes to that of the reflected light from an arbitrary position in the object. Then the measuring light and the reference light are combined by the combining means, and the intensities thereof are detected by the light detector.
In order to obtain a one-dimensional tomographic image, an interference strength waveform according to the reflectance distribution along the same axis as the direction of travel of the measuring light by scanning the optical path length of the measuring light according to the measuring area. That is, a reflected light intensity distribution according to the structure in the direction of depth of the object to be measured can be obtained. Further, when the projecting position of the measuring light applied to the object is one-dimensionally scanned in a direction perpendicular to the optical axis by the use of a deflecting means or a physical means, a two dimensional tomography representing a reflected light intensity distribution can be obtained. Further, when the projecting position of the measuring light is two-dimensionally scanned in directions perpendicular to the optical axis, a three dimensional tomography representing a reflected light intensity distribution can be obtained.
In the above OCT system, a tomographic image is obtained by changing the optical path length of the reference light, thereby changing the measuring position (the depth of measurement) in the object. This technique is generally referred to as “TD-OCT (time domain OCT)”. More specifically, in the optical path length adjusting mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 6(1994)-165784, an optical system which collects the reference light emitted from the optical fiber on a mirror is provided and the optical path length is adjusted by moving only the mirror in the direction of the optical axis of the reference light. Further, in the optical path length changing mechanism for the reference light disclosed in Japanese Unexamined Patent Publication No. 2003-139688, the reference light emitted from the optical fiber is turned to parallel light by a lens, the reference light in the form of parallel light is collected and caused to enter the optical fiber again by an optical path length adjusting lens, and the optical path length adjusting lens is moved back and forth in the direction of the beam axis of the reference light.
Whereas, as a system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SD-OCT (spectral domain OCT). In the SD-OCT system, a tomographic image which is one-dimensional in the optical axis is formed without physically scanning the optical path length, by dividing broad band, low coherent light into measuring light and reference light by the use of an interferometer as in the above-described TD-OCT system, substantially equalizing the measuring light and the reference light to cause them to interfere with each other, decomposing the interference light into the optical frequency components, measuring the intensity of the interference light by the optical frequency components by an array type detector and carrying out a Fourier analysis on the obtained spectral interference waveforms by a computer. As in above-described TD-OCT system, a two-dimensional or a three-dimensional tomographic image can be obtained by scanning the projecting position of the measuring light in directions perpendicular to the optical axis.
As another system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an optical tomography system for obtaining an optical tomographic image by measurement of SS-OCT (swept source OCT) (refer to “Motion artifacts in optical coherence tomography with frequency-domain ranging”, S. H. Yun et al., OPTICS EXPRESS, Vol. 12, No. 13, pp. 2977-2998, 2004.). The SS-OCT system employs a light frequency tunable laser as a light source. The high coherence laser beam is divided into measuring light and reference light. The measuring light is projected onto the object and the reflected light from the object is led to the combining means. The reference light is led to the combining means after it is made substantially equal to the measuring light in the optical path length to cause the measuring light and the reference light to interfere with each other, and the measuring light and the reference light are combined by the combining means. The intensity of the combined light is detected by an optical detector. The intensity of the interference light is measured by the frequency component by sweeping the frequency of the light frequency tunable laser and a one-dimensional tomography in the optical axis is formed without physically scanning the optical path length by Fourier-transforming the spectral interference waveform thus obtained with a computer. As in above-described TD-OCT system, a two-dimensional or a three-dimensional tomographic image can be obtained by scanning the projecting position of the measuring light in directions perpendicular to the optical axis.
There has been investigated application of the optical tomography system of each of the systems described above to a measurement in an organic body by a combination of an endoscope. In the optical tomography system of each of the systems described above, a tomographic image along a certain surface of the object is generally obtained and it is necessary to at least one-dimensionally scan the measuring light beam in the object in perpendicular to the optical axis for this purpose. As a means for effecting such a light scanning, there has been known, as disclosed in Japanese Unexamined Patent Publication No. 4(1992)-135550, an optical probe having a tubular outer envelope and having a function of deflecting light emitted from the peripheral surface thereof in the direction of circumference of the outer envelope. More specifically, the optical probe comprises an inserting portion (outer envelope) which is inserted into the sample, a rotatable hollow shaft which is inserted inside the outer envelope, an optical fiber which is passed through the shaft, and a light deflecting element which is connected to the front end portion of the shaft to be rotated together therewith and deflects light radiated from the front end portion of the optical fiber in a direction of circumference of the outer envelope.
When an optical probe such as disclosed in Japanese Unexamined Patent Publication No. 4(1992)-135550 is inserted into an organic body, a stress change or a temperature fluctuation due to a local pressure change or a local bending of the optical fiber cannot be avoided. There has been well known the fact that when an outer pressure or a strain or a temperature change is imparted to a fiber in an interferometer which measures the interference by the use of the fibers, the refractive index of the fiber or the physical length of the fiber changes and the optical path length in the fiber changes up to 10 times the wavelength. Although, in the TD-OCT system, the difference hardly affects the resolution of the measured data since the difference is sufficiently small because the optical path length of reference light is scanned at high speed to obtain a caustic line of the interference signal. However, in the SS-OCT system or the SD-OCT system described above, strain or delete of information can be generated if the optical path length changes during measurement since the interference waveform of light is measured in the wavelength of the range with the optical path length of the reference light fixed in the SS-OCT system or the SD-OCT system.
Specifically, in the SS-OCT system, since the optical path length of one light shifts and the phase of the light shifts while the wavelength of the light source is scanned, the place shifted from the original place is observed after Fourier transform is observed, which deteriorates the resolution. Further, in the SD-OCT system, where the signals are integrally obtained, if the optical path length changes in a time shorter than the time for which the shutter is opened, there gives rise to a problem that the signals are averaged and the S/N ratio of the signal deteriorates.